1. Field of the Invention
The present invention relates to a mobile communication system, a radio base station, an operation control method and a program thereof, and more specifically to a mobile communication system including the ALL-IP (Internet Protocol) network.
2. Related Art
As the mobile communication system has been widespread, loads on a communication common carrier such as increased frequency usage fee and cost of communication equipment have been controversial. On the other hand, as communication means have got more and more involved in the Information Technology, integration of the mobile communication network and the IP network has been drawing attention. The ALL-IP network has an effect of reducing the cost of equipment. Therefore, more and more businesses are thinking about using the ALL-IP network for their mobile communication network.
The CDMA (Code Division Multiple Access) mobile communication system used as a mainstream of the third generation mobile communication system or the OFDMA (Orthogonal Frequency Division Multiple Access) mobile communication system that is a candidate for the next generation mobile communication system has a feature of providing a stable quality without leading any instantaneous interruption by causing a mobile node to communicate with a plurality of base stations by soft handover when the mobile node moves between cells. As the IPv6 (Internet Protocol Version 6) is introduced, however, the number of IP addresses that can be allocated significantly increased. If a fixed IP address is allocated to a mobile node and the ALL-IP mobile communication network is realized in such circumstances, the IP technology for implementing soft handover between base stations each of which is present on a different IP network is needed. Good consideration on such IP technology has yet to be made.
For example, if a mobile communication system and an IP network are connected for realizing an IP communication between a mobile node (MN) and a correspondent node (CN) by using a fixed IP address with the fact that the MN is moving transparent to a user with a conventional art, the CN and the MN cannot be connected via a plurality of Base Transceiver Stations (BTSs), which may lead instantaneous interruption or packet loss during a hand over of the MN.
Further, the MN excessively issues a Binding update message at a boundary between cells. A large amount of transferred data is present between the IP networks, if a metric (an indicator representing the number of Hops or the bandwidth) from a Home Agent (HA) to a link to be transferred is so large. That may lower the performance of the entire network.
Such a conventional mobile communication system will be described in detail with reference to FIGS. 4 and 5.
FIG. 4 is a diagram showing a configuration of a conventional mobile communication system. Referring to FIG. 4, a CN 3 is communicating with a MN 61. Here, the mobile communication system is connected with an IP network 1, the CN 3 is a correspondent node that is present on a link 2 connected with the IP network 1, and the MN 61 having a fixed IP address is present in a cell 31 that is formed by a BTS 21 present on a link 11. The communication between the CN 3 and the MN 61 is entirely performed by IP including the communication between the BTS 21 and the MN 61. As the MN 61 moves near to a cell 32, the MN 61 tries to perform handover to a BTS 22 in the cell 32.
After the handover, the MN 61 needs to communicate with the CN 3 via a link 12 to communicate with the BTS 22 that forms the cell 32, but the CN 3 does not recognize that the MN 61 has moved and still tries to communicate with the MN 61 via the link 11. Consequently, communication between the CN 3 and the MN 61 is disconnected.
The mobile IP that is optimized for the mobile communication environment causes the MN 61 to informing a Binding update message indicating that the MN 61 is to do handover to a HA 41 that is set on the link 11 so that, after the handover, the HA 41 transfers data sent from the CN 3 to the link 11 to the MN 61 that is present in the cell 32 of the BTS 22 that is connected to the link 12. In such a manner, the mobile IP can keep communication between the CN 3 and the MN 61 without recognizing the fact that the MN 61 has moved being transparent to the CN 3.
Next, a handover method in such a conventional mobile communication system will be described in detail with reference to FIGS. 4 and 5. FIG. 5 is a sequence diagram showing operations of the mobile communication system of FIG. 4. Referring to FIG. 5, the procedure for the MN 61 to do handover from the cell 31 to the cell 32 will be described.
(1) The MN 61 performs the IP communication with CN 3 over the BTS 21 in the handover source cell 31 (step 201). Then, the BTS 21 converts a received IP packet to radio data (step 202). (It is shown as a state A in FIGS. 4 and 5.)
(2) When the MN 61 moves to the boundary of the cell 32 (step 203), the handover to the cell 32 is implemented in the procedure shown below.
(3) The MN 61 obtains a Care of Address (CoA) which is address information of the link 12 in the cell 32 to which the MN 61 is to be connected, via the BTS 22 (step 204).
(4) The MN 61 sends the Binding update message including the obtained CoA to the HA 41 (step 205), and receives data from the cell 32.
(5) The HA 41 that receives the Binding update message from the MN 61 encapsulates the IP packet data destined to the MN 61 and transfers the data to the BTS 22 that is specified by the CoA included in the Binding update message via the IP network 1 (via a path 51) (step 207).
(6) The BTS 22 that receives the transferred data destined to the MN 61 from the HA 41 releases the encapsulation and sends the transferred data as radio data to the MN 61 (step 208). (It is shown as a state B in FIGS. 4 and 5)
As such, in the conventional mobile communication system, data is transferred from the HA 41 to the handover destination BTS 22 during the handover. Therefore, even when the MN 61 moves in the cell 31 or in the cell 32, the CN 3 can keep communication with the MN 61 without considering the current position of the MN 61. A conventional handover method, however, has problems below.
The first problem is in that a packet loss (loss of data) may occur when the MN 61 moves to another link. This is because that the CN 3 and the MN 61 cannot communicate with each other during a period since the MN 61 cannot receive data from the handover source BTS due to its movement to another link until the MN 61 detects a CoA of the link, and a period since the MN 61 tells the HA 41 that the MN 61 has moved and until the HA 41 starts transfer. To address the problem, a method in which the HA 41 transfers data to cells adjacent to the handover source cell 31 in advance (in FIG. 4, the cell 32) has been introduced (for example, see Japanese Patent Laid-Open No. 2004-135178). That has an effect of avoiding a packet loss, but radio data is also sent to unnecessary cells. Therefore, the method has a problem in that radio resources cannot be effectively used. If there are a plurality of adjacent cells, the amount of data to be transferred by the HA 41 increases. That may degrade the performance of the networks around.
In another addressing method that is also examined, the HA 41 buffers data from the CN 3 before the MN 61 does handover, and when the MN 61 completes the handover to the BTS 22 that forms the cell 32, the HA 41 starts transferring the data buffered therein to the MN 61 on the link 12. With the addressing method, no packet loss occurs during the handover, but instantaneous interruption occurs in communication with the CN 3 until the processing of the handover of the MN 61 ends. In addition, the HA 41 may try to collectively send a buffered large amount of data. Therefore, the large amount of data flows into the IP network 1, which may lower the performance of the entire network.
The second problem is in that if the MN 61 moves from the link 11 to a link with a larger metric, delay may occur during the data transfer from the HA 41 to the MN 61 or transfer of a large amount of data may lower the performance of the entire network.
In order to solve the problems, Japanese Patent Laid-Open No. 2006-115119 describes such that when the MN 61 performs handover, it sends to the handover source BTS 21 the Binding update message including address information of the link 12 that is connected with the handover destination BTS 22, so that the data is sent from the handover source BTS 21 to the MN 61, while the same data is transferred from the handover source BTS 21 to the handover destination BTS 22, so that the data is sent from the BTS 22 to the MN 61. Accordingly, the MN 61 receives data from both the handover source BTS 21 and the handover destination BTS 22, and enters in the soft handover state. Therefore, the MN 61 can perform the handover without causing instantaneous interruption of the communication or packet loss.
As mentioned above, the data is transferred from the handover source BTS 21 to the handover destination BTS 22 during the handover. The adjacent BTS can be arranged in a configuration with a small metric even on the IP network, however, even if data is transferred during the handover, the delay may be reduced. In addition, possibility that performance of the entire network is lowered can be eliminated.
Furthermore, Japanese Patent Laid-Open No. 2006-115119 discloses that CN 3 uses a path via the handover destination BTS 22 as a path of the IP packet that is sent to the MN 61 by sending the Binding update message from the MN 61 to the CN 3 when the handover completed.
As such, in the technique described in Japanese Patent Laid-Open No. 2006-115119, data is transferred from the handover source BTS 21 to the handover destination BTS 22. If a metric between the link 11 and the link 12 is large, a delay occurs until the data transferred from the BTS 21 arrives at the BTS 22 and even the MN 61 is in the soft handover state, the radio data sent from the BTS 22 may be merely taken as a multi-path interference noise. A delay occurs until the BTS 22 that receives the transferred data from the BTS 21 releases encapsulation and sends the data as radio data and even the MN 61 is in the soft handover state, the radio data sent from the BTS 22 may be merely taken as a multi-path interference noise.